Signaling Ranges and Characteristics
network communications are made via a large range of
electromagnetic wave frequencies using "wires" or
"wireless" transmissions. An electromagnetic
wave is the physical form of energy described by the
electromagnetic spectrum. The spectrum starts at zero
oscillations, rises through the range of frequencies that can be
perceived by human senses through the various forms of light, up to the
frequencies called X-rays and gamma rays.
The rates of frequency are measured in Hz - Hertz. This is directly related to the wavelength, which can vary from billionths of a meter to several meters. For whatever reasons, Hz are usually used for lower frequency bands and wavelengths are usually used when talking about higher frequency phenomena such as light. The standard names and ranges/bands are given in the following diagram.
|Notice how the visible range is very very small.
We now need to quickly define a few terms which you have hopefully seen
Now we need to state one basic principle.
The following tables contain some of the most important signal characteristics for low and high frequencies.
|FCC regulated due to the above|
|potential for tightly focused transmission|
For example, light doesn't penetrate even the slimmest of opaque materials. But it is very capable of transmitting data at high rates. Radio waves tend to be rather omni-directional, but not very frangible. We will make use of these concepts to compare different transmission media.
EMI or distortion can impact TV reception, cordless phones, airplane crashes, death caused by medical equipment failure. Copper wire is extremely susceptible to EMI. Coax's outer shielding offers some additional protection. This is also true for STP - Shielded Twisted Pair. UTP is fairly vulnerable, but the twisting helps reduce its impact. Fiber optic cabling is the best when dealing with high EMI environments.
Radio Transmission. There can also be radio frequency interference. What is done when using Spread Spectrum Radio transmissions is that only certain frequency ranges are used. The FCC has allocated the 902 - 928MHz and 2.4 - 2.4835GHz bands of the electromagnetic spectrum for industrial, scientific and medical (ISM) use. They are referred to as the ISM bands. The uses of these bands is largely unregulated except to establish guidelines for electrical and electronic devices that use these bands.
Fortunately, these ISM bands provide sufficient bandwidth to compete with wire based LANs. They also tend to be relatively inexpensive since bands are not licensed and manufacturers can provide hardware for less money than they can for dedicated frequency products.
Unfortunately, radio signals are not capable to full-duplex communication on a single frequency, you are either sending or receiving, but not both simultaneously. For example, think of walkie-talkies and the push to talk necessities. So when you hear a particular bandwidth described for wireless you are likely to need to divide it in half in order to compensate for using some frequencies in each direction. But the actual bandwidths depend on a lot of other things such as what sort of specifications you are using, which we will get into in much more detail in later webpages.
Effective throughputs can really be quite small and usually much smaller than for wire based networks. Thus, radio based connectivity could easily be a relative bottleneck in current networking environments. In addition, the absence of FCC licensing to give dedicated frequencies requires limiting wattage so that reach is somewhere between 600 and 800 feet. There are also other issues of competition for this range within zones that may greatly reduce its effectiveness.
Single band radio is essentially the opposite of spread spectrum and uses a single channel. This signal is usually sent in the microwave range which are actually high frequency radio waves. Waves at the lower end of the microwave range behave much like radio waves, at the high end they behave much more like light.
To use a dedicated frequency you need FCC licensing. This technology was pioneered by Motorola who obtained exclusive rights to the 18 - 19 GHz band for all the major metropolitan markets in the US. Motorola acts as an agent to the FCC for any customers wanting to use this technology.
Firms that make sue of this maintain their existing wire based LAN backbones, hubs and software drivers. Transmission voltage is approximately 25 milliwatts which is too low to cause health concerns in metropolitan areas. The wattage, in conjunction with the relatively frangible microwave signal, limits the effective range to 140 feet of open air and 40 feet with sheetrock wall obstructions.
The gross bandwidth is approximately 15Mbps. Modifying this due to typical Ethernet considerations reduces the effective yield to around 5.5Mbps, comparable to wired Ethernet LANs.
Infrared. The infrared spectrum operates between the visible part of the electromagnetic spectrum and the shortest microwaves. Infrared is actually a form of light which cannot penetrate opaque solids but does reflect off them.
Infrared can be used in direct and/or diffuse form. Direct infrared is typically used in household electronics with remote control devices which must be pointed at the device to be controlled. This is the same when applied to LANs. There has to be a line-of-sight connection.
Diffuse infrared scatters omnidirectionally. The intent is to bounce signals off ceilings and walls so that the transmit/receive device doesn't need to be in line-of-sight.
Infrared communications are not capable of penetrating even the least dense of opaque solids and thus don't require FCC regulation. But the FCC establishes specifications about the devices using the infrared spectrum. Lin-of-sight communications are severely limiting in most environments, particularly offices. Thus in offices, diffuse infrared is necessarily preferred to direct. But considering the highly frangible nature of the medium the effective range is extremely small, less than 100 feet. In addition the throughput rates are extremely modest.
Laser. You can think of laser communications within a LAN as single mode fiber optic communication without the fiber optic. The cost generally precludes its use on a station by station basis. It's better when applied in ways similar to direct infrared. Essentially, it is best used to interconnect access units that are connected in some other way to stations. They are best when mounted away from people, as near to the ceiling as possible. This makes them much less likely to cause eye injuries or to be disrupted. They can also be used to bridge gaps such as parking lots. This sort of implementation usually costs less than paying for routers and leased lines.